the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Apr 2025
Projections of actual and potential evapotranspiration from downscaled high-resolution CMIP6 climate simulations in Australia
Abstract. Quantifying the impact of climate change on actual and potential evapotranspiration (AET and PET) is essential for water security, agriculture production and environmental management. AET and PET are strongly influenced by local factors such as topography, land cover and soil moisture, which limits the usability of global climate models for their projections. Here, we dynamically downscale CMIP6 models using CCAM to a 10 km resolution over Australia and derive AET and PET at a daily time step using the Morton method and project future changes under SSP126, 245 and 370.
The performance of observation- and downscaled climate model-based AET is assessed against measured AET from 26 OzFlux sites in Australia. We show that high resolution downscaled climate models can provide reasonably accurate estimations of AET in Australia, with an ensemble mean error of 17 % for historical period 1981–2010. This compared favourably to observation- and reanalysis-based products, which reported mean errors ranging from 15.7 % – 44 %.
Annual average end-of-century AET projections for low and intermediate emission scenarios (SSP126 and SSP245) show a decrease of -4.5 % and -3.5 % respectively in Australia, while under high emissions (SSP370) AET was projected to increase by 1.8 %. In contrast, PET was projected to increase by 5.0 % for SSP126, 8.4 % for SSP245 and 11.5 % for SSP370.
Using random forest model, we show that the primary controlling factors for changes in AET are precipitation and solar radiation, and solar radiation and maximum temperature for PET. Our results offer new insights into future AET and PET changes estimated using downscaled CMIP6 simulations with implications for agriculture, water supply and natural resources management.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-498', Anonymous Referee #1, 22 May 2025
The manuscript presents a very thorough and broad assessment of AET and PET datasets and projections for Australia. The introduction, analysis and discussion give a good overview of the performance of the available AET and PET products. The paper is well written, clearly structured and I appreciate that the limits of the products and the results are clearly stated. Here are my comments on the manuscript:
- First, the abstract would benefit from more plain language. While abbreviations like AET and PET are defined, CCAM is introduced without explanation. Removing it or adding a brief description of what CCAM refers to and why it is used would make the abstract more accessible to non-specialist readers.
- The introduction is very thorough but also very long, and the study's aims only become apparent at the end. The mention of “old observational datasets” seems abrupt and lacks sufficient context. Clarifying how these datasets relate to the objectives would improve readability and make it easier to follow.
- The initial visual comparison of datasets across Australia is a helpful starting point, but it would be even more informative if it were supplemented with a quantitative assessment of spread or uncertainty between the datasets, e.g. a map of the model spread. Specifically, identifying regions with the largest disagreement among models would highlight areas where confidence is lower and further improvement is needed.
- Adjusting the order of SSP scenarios in Figure 6 to the order used in Figures 7 and 8 would make it more intuitive to interpret the results. Also, I would include the caption entirely for Figure 8, even if it is the same as for Figure 7, so that the figure can stand on its own and readers don’t have to jump between the two.
- Lastly, regarding the drivers of change, CMIP6 models usually include LUC in their scenarios. It would be interesting to discuss if some of the changes you identify can be tied to LUC instead of CC. What do you think?
Citation: https://doi.org/10.5194/egusphere-2025-498-RC1 -
AC1: 'Reply on RC1', Hong Zhang, 12 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-498/egusphere-2025-498-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-498', Anonymous Referee #2, 23 Jul 2025
The manuscript uses dynamically downscaled CMIP6 datasets as input to estimate Actual Evapotranspiration (AET) and Potential Evapotranspiration (PET) under historical and various future climate scenarios. It also employs a random forest approach to identify key driving factors influencing projected changes in AET and PET. In addition, the study evaluates multiple datasets against site observations, providing a valuable reference for selecting appropriate AET or PET products. I appreciate the authors' efforts in conducting these evaluations and projections. However, several issues should be addressed before the manuscript is suitable for publication:
1. Justification of CMIP6 model selection
The authors should clarify the rationale behind the selection of specific CMIP6 models and ensembles. Why were these models chosen? Do other CMIP6 models not provide the relevant variables? A brief explanation would help readers understand the basis for the selection.
2. Consistency of projections among models
The manuscript uses the mean values from multiple CMIP6 projections. However, it is unclear whether the individual models indicate consistent changing trends (e.g., all showing an increase or decrease). Are there any models that suggest an opposite direction of change, which may have been masked by averaging? This should be discussed to provide a clearer picture of the uncertainty and variability in the projections.
3. Definition of CCAM
The abstract mentioned CCAM without defining it. The full name should be provided upon first mention.
4.Improving logical flow in the Introduction.
The introduction could benefit from improved coherence. While the authors have evaluated both AET and PET, the transitions between topics are sometimes abrupt. For instance, around line 55, the discussion shifts from AET to PET and then back to AET, which disrupts the logical flow. Strengthening the narrative structure would enhance readability
5.Figure 1 caption clarity
The caption for Figure 1 does not explain the meaning of the solid and dashed lines, which makes it difficult to interpret the boundaries of the eight Natural Resource Management (NRM) regions. Although the regions are numbered, a clearer description of the line styles is needed.
Citation: https://doi.org/10.5194/egusphere-2025-498-RC2 -
AC2: 'Reply on RC2', Hong Zhang, 12 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-498/egusphere-2025-498-AC2-supplement.pdf
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AC2: 'Reply on RC2', Hong Zhang, 12 Aug 2025
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